Sensor arrangement

ABSTRACT

A sensor arrangement is described comprising a sensor housing (28), a first sensor (30) sensitive to a downhole parameter and carried by the housing (28), a second sensor (32) sensitive to the same downhole parameter as the first sensor (30) and carried by the housing (28), the second sensor (32) being spaced apart from the first sensor (30) by a fixed distance D in the axial direction of the housing (28), and a control unit (38) operable to monitor the outputs of the first and second sensors (30, 32) to ascertain information relating to the position, movement and/or related information of the housing (28). A related operating method is also described.

This invention relates to a sensor arrangement, and in particular to a sensor arrangement adapted for use in a borehole to enable downhole tool position or related information to be obtained.

Where a borehole is being drilled in an earthen formation, for example for subsequent use in the extraction of hydrocarbon materials, it is usual to use a bottom hole assembly including a drill bit to drill, scrape or abrade the formation material to extend the length of the borehole, the bottom hole assembly being carried at an end of a drill string made up of a series of lengths of drill pipe connected in an end-to-end fashion. As drilling progresses, the drill string is moved further and further into the borehole and, periodically, the drill string is extended in length by attaching an additional drill pipe thereto. Commonly, rather than attach a single additional length of drill pipe to the drill string, preassembled groups or stands of drill pipe, for example including three drill pipe lengths, may be attached in a single operation.

In order to allow good control over the drilling operation it is desirable to know the position of the bottom hole assembly. A rough position value can be obtained by keeping count of the number of drill pipes present in the drill string as the drill pipes are of a known length. However, as each drill pipe is of a relatively great length, use of this information alone only provides a very approximate guide to the position of the bottom hole assembly. By supplementing this information with a measurement of the length of the drill string at the surface and yet to enter the borehole, a more accurate indication of position may be obtained. Such so-called driller's depth measurements are relatively inaccurate as the true length of the drill string can vary from its calculated length for a range of reasons, introducing inaccuracies into the measurement of the depth or length of a borehole. By way of example, temperature variations along the length of the borehole can result in significant levels of thermal expansion of the drill pipe material, and loads on the drill string (especially in vertical or near vertical borehole sections) can result in stretching thereof. In addition, the addition of weight on bit loadings to enhance drilling can result in compression or shortening of the drill string, as can the effect of buoyancy of the bottom hole assembly. Buckling of the drill string also impacts upon the accuracy of position information obtained using these types of measurement. By way of example, for a deep well, these effects may result in a discrepancy of over 30ft in the actual length of the borehole as compared to the measured length achieved using this technique.

During drilling, changes in the nature of the formation material through which the borehole is being formed can be noted by the operator who will often make a note of the driller's depth measurement at the point at which such changes are encountered. However, as the driller's depth measurement is relatively inaccurate, the information obtained relating to the occurrence of such features can also be misleading.

It is known, whilst drilling is interrupted to allow the addition of further lengths of drill pipe to the drill string, to undertake measurement while drilling surveys of the borehole. Undertaking these additional measurement operations is relatively complex and time consuming, typically requiring the bottom hole assembly to be lifted from the bottom of the borehole and held motionless whilst the survey is undertaken. There is a risk that if the steps or processes used to undertake such a survey are not followed or performed properly, the information obtained during the survey may be of reduced accuracy and so the survey may need to be repeated. Obviously, this is undesirable. Also, holding the bottom hole assembly motionless with drilling fluid circulation interrupted for a significant period of time as is required to undertake such surveys is generally undesirable.

The approach that is often used to provide position information is to periodically conduct a three axis gravitational and magnetic survey as mentioned above to obtain accurate position information and other information about the borehole, and to supplement this with information relating to the length of the drill string that has been added since the last survey was undertaken to derive an overall position.

Where such surveys provide information relating to changes in formation material, then it may be desired to relate the driller's depth based information obtained during drilling to the information obtained during the survey. However, the inaccuracies in the driller's depth readings can make this very difficult to achieve.

As the position information may be used in the control of drilling to ensure that the borehole substantially follows a predetermined path, positional errors of the magnitude discussed above can have series negative effects, for example leading to the borehole failing to pass through a selected reservoir. There can also be legal consequences to the failure of the borehole to follow a predetermined path.

It is an object of the invention to provide an arrangement whereby position and related information can be obtained substantially in real time whilst drilling is being undertaken.

According to the present invention there is provided a sensor arrangement comprising a sensor housing, a first sensor sensitive to a downhole parameter and carried by the housing, a second sensor sensitive to the same downhole parameter as the first sensor and carried by the housing, the second sensor being spaced apart from the first sensor by a fixed distance in the axial direction of the housing, and a control unit operable to monitor the outputs of the first and second sensors to ascertain information relating to the position, movement and/or related information of the housing.

The downhole parameter preferably comprises temperature information. The use of temperature information is advantageous in that, in addition to allowing position or movement information to be derived, it can allow other information to be derived. By way of example, temperature variations occur when drilling is interrupted to allow the connection of additional drill pipe to the drill string. The making of such connections can thus be detected downhole using the sensor arrangement, and information derived in this manner may be used, for example, in correlation of the pipe tally to the downhole measured depth.

The first and second sensors each conveniently comprise a sensor array or a plurality of individual sensor units each located at the same axial position but angularly spaced apart around the housing. Accordingly, localised temperature variations can be detected by the individual sensor units of each array detecting the temperature variations in turn as the housing is rotated. By way of example, a temperature hot spot will appear to wash around the housing as the housing is rotated and along the housing as the housing moves axially. In normal use, the temperature variation will appear to follow a helical path relative to the housing, with the pitch of the helix being related to the axial speed of movement of the housing.

Conveniently, the housing includes one or more further sensors, each of which conveniently takes the form of a sensor array or a plurality of individual sensor units of a form similar to the first and second sensors. By way of example, four or more such sensors may be provided, each of which may be made up of a row or ring of individual sensor units.

The first and second sensors may be spaced apart by a small distance, for example being spaced apart from one another by 10 mm or less. Alternatively, they may be spaced apart from one another by larger distances, for example by 10s or 100s of feet. Obviously, they may be spaced by distances between these values.

It will be appreciated that the first and second sensors, and further sensors if provided, may together form a panoramic thermal camera, providing a thermal image of the area surrounding the housing, the image being representative of the temperature surrounding the full 360° circumference of the housing. The resolution of such a camera is dependent upon the number and size of the individual sensor units provided, and upon the sensitivity of each of the sensor units. The control unit is preferably arranged to undertake image processing to monitor the passage of a hot spot or other thermal feature through the area being imaged by the panoramic thermal camera, and thereby determine the axial progression of the housing and monitor angular movement of the housing. As mentioned above, where a hot spot or other thermal feature follows a helical path through the panoramic image obtained using the sensors, the pitch of the helix is related to the axial speed of movement of the housing.

The housing may be incorporated into or included in a bottom hole assembly, providing position information or the like relating to the position of the bottom hole assembly.

The sensors or sensor units conveniently comprise thermoelectric generators which may serve, in use, as thermal cameras detecting temperature variations, as mentioned hereinbefore. The thermoelectric generators may be arranged to detect a temperature difference between the temperature of a wall of a tubular member carrying drilling fluid in a downhole direction and a wall of the temperature of part of the housing.

In use, as the sensor arrangement moves along the length of the borehole, the control unit monitoring the outputs of the first and second sensors can compare the outputs thereof and ascertain when the second sensor is aligned with a feature the position of which is fixed and which has previously been identified by monitoring the output of the first sensor when the feature is aligned therewith. When this position is reached, it is known that the sensor arrangement has moved by a distance equal to the axial separation of the first and second sensors. By continuously or periodically monitoring the outputs of the first and second sensors in this fashion, the movement of the sensor arrangement, and hence of the bottom hole assembly or the like to which it is mounted or of which it forms part can be monitored, allowing the position thereof to be measured. By using the position information in association with time information, speed or rate of penetration information may also be obtained. It will be appreciated that the measured depth of the bottom hole assembly, and hence length of the borehole, may be determined with improved accuracy, avoiding the inaccuracies arising from the factors mentioned hereinbefore. By measuring the position of the bit, downhole tools and sensors, etc, relative to the housing prior to deployment, the positions and orientations of these components can be monitored with a good degree of accuracy, in use.

The position and/or related or other information derived using the sensor arrangement such as connection detection as mentioned hereinbefore, and enhanced environmental awareness, may be supplied to an operator at the surface and/or may be supplied to a downhole located drilling control unit to allow drilling to be undertaken autonomously.

Whilst the position information obtained using the sensor arrangement may be used by itself, the information may be used in conjunction with information derived from a traditional measurement whilst drilling survey to enhance the quality of the information attained thereby, if desired.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view illustrating a sensor arrangement in accordance with an embodiment of the invention, in use; and

FIG. 2 is a diagrammatic view illustrating the sensor arrangement in greater detail.

Referring to the accompanying drawings, FIG. 1 illustrates, diagrammatically, a drilling apparatus 10 in use in the formation of a borehole 12 in the ground. The drilling apparatus 10 comprises, in this example embodiment, a surface located rig 14 that is arranged to support a drill string 16 that extends along the length of the borehole 12. The drill string 16 is made up of a series of individual drill pipes 18 that are connected to one another in an end-to-end fashion. Located at the lower end of the drill string 16 is a bottom hole assembly 20 including a drill bit 22 and a number of other components. The rig 14 is arranged to drive the drill string 16 for rotation, in use, and to supply drilling fluid under pressure through the drill string 16 to the lower end of the borehole 12 from where it is delivered through nozzles to the borehole 12 and serves to clean and cool parts of the drill bit 22 and to carry formation material drilled, cut or the like by the drill bit 22 along the borehole 12 to the surface, the drilling fluid flowing upwardly along the borehole within an annular passage 24 defined between the exterior of the drill string 16 and the wall of the borehole 12.

The bottom hole assembly 20 includes, in accordance with the invention, a sensor arrangement 26. As shown diagrammatically in FIG. 2, the sensor arrangement 26 comprises a housing 28 within which is located a first sensor 30 and a second sensor 32. The first and second sensors 30, 32 are spaced apart from one another in the axial direction of the housing 28 by a fixed distance D. The distance D may be as small as a few millimetres, or may be as large as 100s of feet, or may be between these values. The sensors 30, 32 are both sensitive to the same parameter as each other, in this case to temperature. By way of example, as illustrated, each of the sensors 30, 32 may take the form of a thermoelectric generator unit located between an outer wall of the housing 28 and an inner tubular member 34 located concentrically within the housing 28 and through which the drilling fluid from the drill string 16 is delivered to the remainder of the bottom hole assembly 20 located downstream of the sensor arrangement 26, the sensors 30, 32 being operable to produce an electrical output in the event of there being a temperature differential between the parts of the outer wall of the housing 28 and the inner tubular member 34 immediately adjacent thereto.

Each sensor 30, 32 may comprise an array in the form of a plurality of individual sensors units 30 a, 32 a disposed in respective rings encircling the inner tubular member 34. The sensors 30, 32 may thus together act as a panoramic thermal camera providing an output in the form of a 360° panoramic thermal image of the temperature of the housing adjacent to the sensors 30, 32, and of the fluid within the surrounding parts of the borehole. A control unit 38 is provided to monitor and compare the outputs of the first and second sensors 30, 32, identifying when a temperature feature or spike identified by the first sensor 30 is subsequently identified by the second sensor 32 after movement of the housing and bottom hole assembly 20 by distance D. The control unit 38 may perform an image processing function, tracking the progression of a hot spot or other thermal feature in the panoramic thermal image as it passes through the thermal image. Typically, a hot spot or thermal feature at a fixed location will appear to follow a helical path through the thermal image, and it will be appreciated that the pitch of the helix will be related to the axial speed of movement of the housing.

The sensors 30, 32 may be located relatively close to one another, for example at a spacing of, say 10 mm or less, or they may be spaced apart by a greater distance. By way of example, they could be spaced apart by 10s or 100s of feet as noted above. By appropriate programming of the control unit 38, the spacing of the sensors 30, 32 may be anywhere between these values.

Whilst the accompanying drawings illustrate an arrangement including just two sensors 30, 32, each of which is made up of a number of individual sensor units 30 a, 32 a, it will be appreciated that a greater number of sensors may be provided. The resolution of the panoramic thermal camera is dependent upon the number, dimensions and spacings of the individual sensor units 30 a, 32 a, and upon their sensitivity, and these can be adjusted to suit the application in which the arrangement is being used.

In use, rig 14 is operated to supply drilling fluid through the drill string 16 towards the bottom hole assembly 20 and to rotate the drill string 16 and the bottom hole assembly 20 carried thereby. The drill bit 22 is rotated to scrape or cut formation material from the end of the borehole 12, extending the length thereof, and the material cut or scraped from the formation is carried along the borehole 12 towards the surface through the annulus 24 by the return flow of drilling fluid.

Throughout the drilling operation, the control unit 38 connected to the outputs of the sensors 30, 32 of the sensor arrangement 26 operates to monitor the temperature differentials between the parts of the outer wall of the housing 28 and the inner tubular member 34 adjacent the sensors 30, 32. The temperature of the inner tubular member 34 is largely dictated by the temperature of the drilling fluid supplied by the rig 14 through the drill string 16 and is relatively cool. The temperature of the various parts of the outer wall of the housing 28 will normally be higher than that of the inner tubular member 34 and may vary depending upon a number of factors. By way of example, as the sensor arrangement 26 passes through different parts of the formation material where flows of fluid to or from the borehole 12 may occur, temperature variations may occur. Where the borehole 12 includes a curve that results in the outer wall of the housing 28 bearing against the borehole wall, then frictional heating of parts of the housing 28 can be expected. It will be appreciated that these represent merely examples of conditions that may give rise to detectable temperature changes at specific fixed locations along the borehore 12.

In use, the control unit 38 notes the occurrence of a feature such as a temperature spike or temperature profile as detected by the first sensor 30 and monitors for that same feature to subsequently be detected by the second sensor 32. As mentioned above, this may be achieved using an image processing technique to track the passage of the feature through the panoramic thermal image obtained using the sensors 30, 32 as a panoramic thermal camera. As the first and second sensors 30, 32 are spaced apart axially by a known fixed distance D and it is known that the cause of the detected temperature spike is at a fixed location, when the second sensor 32 detects the feature previously detected by the first sensor 30, it can be ascertained or determined that the sensor arrangement 26, and hence the bottom hole assembly 20, has progressed by a distance equivalent to the fixed distance or separation D of the first and second sensors 30, 32. By noting the time delay between detection of the temperature feature by the first and second sensors 30, 32, a value for the axial velocity of the tool can be ascertained, and by continuously or periodically monitoring the outputs of the sensors 30, 32 in this fashion an accurate determination of the measured depth, ie the length of the borehole 12, can be determined. Alternatively, the axial velocity may be derived from the pitch of the helical path followed by the feature in the panoramic image. In comparing the outputs of the sensors 30, 32, the control unit 38 may be essentially undertaking a pattern matching process.

By using sensors 30, 32 of the type outlined hereinbefore, it will be appreciated that a localised hot spot or other temperature feature at a fixed location will appear to wash around as well as along the housing 28, being detected by the individual sensor units 30 a, 32 a in turn as the housing rotates and by the sensors 30, 32 as the housing moves axially. The arrangement is thus sensitive to both axial and angular or rotary movement.

The sensors 30, 32 are preferably of a type having a very short response time, for example in the region of 20 ms, and are sensitive to small changes in temperature, for example in the region of 1° C. or less, preferably 0.05° C. or less, and more preferably 0.01° C. or less. Accordingly, in use, there is no shortage of temperature spikes or features for the control unit 38 to use in determining when a feature previously detected by the first sensor 30 has subsequently been detected by the second sensor 32. Accurate position or depth measurements can thus be undertaken.

Whilst the sensors 30, 32, and the sensor arrangement 26 as a whole, may take a range of forms, conveniently it is of a form substantially as described in copending British Patent Application No 1909016.6 but modified to include a pair of sets of sensor modules axially spaced apart from one another by a known, fixed distance D. This could be achieved through the use of a pair (or more) of the tools of the type described in British Patent Application No 1909016.6 controlled or used appropriately.

It will be appreciated that by continuously monitoring the progress of the bottom hole assembly 20 using the sensor arrangement 26 in the manner set out hereinbefore, the length of the borehole between the surface and the sensor arrangement 26 can be accurately determined. The output of the sensor arrangement 26 may be supplied to, for example, a surface located controller and used in controlling various drilling parameters, for example to assist in ensuring that the borehole 12 follows a predetermined path. Alternatively, or additionally, the output of the sensor arrangement 26 may be supplied to downhole located control equipment, allowing drilling to be undertaken autonomously.

Whilst the output of the sensor arrangement 26 may be used alone to provide position information, in some situations it may be desirable to use the output thereof in conjunction with the outputs of other sensors and the like. By way of example, where the drilling apparatus 10 includes a conventional measurement whilst drilling survey device, the output of the sensor arrangement 26 may be used to confirm that the conditions are suitable for use of the survey device. Typically, use of such a survey device requires the bottom hole assembly to be lifted from the bottom of the borehole, for rotation to cease, the drill pipe to be reciprocated back and forth to alleviate torque therein and for the pumping of drilling fluid to be interrupted. All of these operations can be detected by the sensor arrangement 26. By way of example, lifting the drill bit from the bottom of the borehole and stopping drilling leads to a reduced pressure differential across the bit, and hence to less heating of the drilling fluid, detectable by the sensor arrangement. Stopping rotation and undertaking axial reciprocating motion can be detected by the sensor arrangement. Torque relief can be detected by sensing the cessation of angular movement of the sensor arrangement 26. The sensor arrangement 26 may thus be used to provide a confirmation that the conditions are appropriate for the survey device to conduct a measurement whilst drilling survey, and may confirm whether or not movement has occurred during such surveys. Similarly, the sensor arrangement 26 can provide confirmation that the procedures undertaken after completion of the survey to allow drilling to recommence have taken place. The use of the invention can thus aid in enhancing the accuracy of the results of such surveys.

Through the use of artificial intelligence or the like, the sensor arrangement 26 may automatically detect the commencement of procedures to add lengths of drill pipe to the drill string and/or commence a survey and provide appropriate outputs to a controller in response thereto. As described hereinbefore, such procedures lead to the occurrence of temperature variations that can readily be detected by the sensor arrangement and used to provide an output indicating that a connection procedure to add a length of drill pipe to the drill string and/or the undertaking of a survey is underway. Furthermore, the invention may allow automated measurement of the distance moved by the housing subsequent to conducting such a survey for use as outlined hereinbefore to provide accurate, potentially real-time position information.

It will be appreciated that as well as providing position and/or rate of penetration information, the sensor arrangement may also be used to detect the occurrence of other conditions such as buckling of the drill string or twist-off conditions. As the sensor arrangement measures temperature, the output thereof may also be used to aid calculation of thermal expansion of the drill string. By programming the tool with information relating to the length of each drill pipe section, and by keeping count of the number of drill pipe sections, and by using the measured temperature information to calculate the level of thermal expansion that will have taken place, it can be determined whether the drill string length calculated by adding together the lengths of the drill pipe sections used and adjusting for thermal expansion is compatible with the measured position to provide an indication of the occurrence of a buckling condition, or the occurrence of squatted pipe or stretched pipe conditions.

Whilst the description hereinbefore is of the use of the sensor arrangement 26 as part of the bottom hole assembly 20, or located adjacent thereto, it will be appreciated that if desired additional sensor arrangements of the type described hereinbefore may be employed elsewhere in the drill string to provide additional information for use in control of drilling. These may be employed in addition to the aforementioned sensor arrangement or as an alternative thereto. Increasing the number of sensor arrangements deployed along the string will improved detection and accuracy of these measurements.

In the arrangement described hereinbefore, the sensors 30, 32 are both highly sensitive to variations in the temperature differentials between the outer wall of the housing 28 and the inner tubular member 34. It will be appreciated that the invention is not restricted in this regard and could make use of sensors of other forms. By way of example, other forms of temperature sensor may be used. Alternatively, sensors sensitive to other parameters could be used. Furthermore, whilst the sensor arrangement 26 described hereinbefore includes two sensors spaced apart from one another by a fixed distance, additional sensors could be provided, if desired, for example providing additional resolution and so allowing enhancements in measurement accuracy to be made.

Although a specific embodiment of the invention is described hereinbefore it will be appreciated that a number of modifications or alterations may be made thereto without departing from the scope of the invention as defined by the appended claims. 

1. A sensor arrangement comprising a sensor housing, a first sensor sensitive to a downhole parameter and carried by the housing, a second sensor sensitive to the same downhole parameter as the first sensor and carried by the housing, the second sensor being spaced apart from the first sensor by a fixed distance in the axial direction of the housing, and a control unit operable to monitor the outputs of the first and second sensors to ascertain information relating to at least one position, movement and related information of the housing.
 2. An arrangement according to claim 1, wherein the housing is incorporated into or attached to a bottom hole assembly, providing position information relating to the position of the bottom hole assembly, and/or is located elsewhere in a drill string.
 3. An arrangement according to claim 1, wherein the downhole parameter comprises temperature information.
 4. An arrangement according to claim 1, wherein the first and second sensors comprise one of thermoelectric generators and thermal cameras operable to produce an output indicative of a temperature difference between parts of the housing and an inner tubular member adjacent the respective sensors.
 5. An arrangement according to claim 4, wherein the first and second sensors together form a panoramic thermal camera.
 6. An arrangement according to claim 5, wherein the control unit undertakes an image processing technique to track the passage of a thermal feature through an image captured using the panoramic thermal camera.
 7. An arrangement according to claim 1, wherein position and/or related information derived by the control unit is supplied to at least one of (a) an operator at the surface and (b) a downhole located drilling control unit to allow directional drilling to lie undertaken autonomously.
 8. A sensing method comprising locating within a borehole a sensor arrangement as claimed in claim 1, and monitoring the outputs of the first and second sensors to identify when a feature previously detected by the first sensor is subsequently detected by the second sensor to determine that the sensor housing has moved by a distance equal to the spacing of the first and second sensors.
 9. A method according to claim 8, wherein the distance information is used to determine an axial velocity of the sensor housing.
 10. A method according to claim 8, and further comprising using the output of the first and second sensors, when undertaking a measurement whilst drilling survey, to confirm that at least one procedural step of the survey has been completed.
 11. A method according to claim 10, and further comprising using the output of the first and second sensors to detect the occurrence of a drill pipe connection procedure.
 12. A method according to claim 8, wherein the distance information is used to determine the occurrence of at least one of buckling, stretching and squat pipe conditions. 